DDN-4630 sec: cleanup grouplist alloc for INTERNAL id upcall

DDN-4630 sec: cleanup grouplist alloc for INTERNAL id upcall

With the INTERNAL identity upcall, we are using supplementary groups
provided by the client, by building an array of gid_t.
Cleanup this group list allocation, and make sure the size returned
matches the actual size of the allocated array.

Fixes: 4515e5365f ("LU-17015 build: rework upcall cache")
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I72cdfc6b76bfd9c2832a5d5e5f72c3aa45cf1efe
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53979
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>